1. Technical Field of the Invention
The present invention relates generally to brushless permanent magnet motors for automobiles, household electrical appliances, and industrial uses.
More particularly, the invention relates to a mufti-phase, brushless, and permanent magnet motor that has an improved structure in which the number of stator poles is less than that of rotor poles.
2. Description of the Related Art
For three-phase AC motors, there are many combinations between the number of rotor poles and the number of stator teeth. Among them, the combination of eight rotor poles and twelve stator teeth, which is generally referred to as 2-3 system, has been widely used. (For example, a reference can be made to Japanese Patent First Publication No. H06-261513.)
FIG. 16 shows a conventional three-phase AC motor in which the 2-3 system is used.
As shown in FIG. 16, the motor includes a rotor 1 and a stator 2. The rotor 1 is configured with a rotor core 6 and eight permanent magnets 3 that are arranged on the surface of the rotor core 6. The stator 2 is configured with a stator core 7, which has twelve teeth 4, and stator windings 5 that are wound around the stator core 7 by a concentrated winding method.
According to the concentrated winding method, the stator core 7 is in a divided form to improve the space factor of the stator windings 5. More specifically, as shown in FIG. 19, each tooth 4 is divided into two parts 4a and 4b, between which the stator windings 5 wound around a bobbin are inserted.
Further, as shown in FIG. 17, with respect to the rotor poles having a pitch corresponding to an electrical angular width of 180°, the stator poles have, at the inner end of the teeth 4, an angular width that corresponds to an electrical angular width a of 120°. Defining the winding factor k as cos((180°−α)/2), then k is equal to 0.866 in this case. Indeed, since there are slot openings formed between the teeth 4, the electrical angular width α is in the range of 100 to 110°; Accordingly, the winding factor k will have a smaller value of 0.766 to 0.82.
In addition, if a distributed winding method is used instead of the concentrated winding method, the electrical angular width α corresponding to the angular width of the stator poles would become 180°, as shown in FIG. 18; the winding factor k would accordingly become equal to 1.
The above-described conventional motor has the following disadvantages.
Since the angular width of the stator poles is small, in terms of electrical angular width, with respect to the pitch of the rotor poles, the torque induced by electric current flowing through the stator windings 5 is small. Moreover, due to the small winding factor k, both the power output and efficiency of the motor are low.
Further, though the space factor of the stator windings 5 is improved by dividing the stator core 7, magneto-resistance increases at the connecting portions between the divided parts of the stator core 7, thus decreasing both the power output and efficiency of the motor.
In addition, the divided structure of the stator core 7 yields a magneto-resistance unbalance in the motor, which causes vibrations and noise of the motor to increase.